Aromatic polyester films have excellent optical characteristics with regard to transparency and optical isotropy. One characteristic which is important to the practical use of aromatic polyester films is the property of easy surface slipperiness, sometimes called the travelling property. Since good slipperiness between a film and its support (e.g., roll), or between adjacent films, are needed in the various stages of processing the films, operability and handleability in each stage can be improved and undesired failures, such as wrinkling of the films, can be prevented from occurring if surface slipperiness is improved.
In imparting easy slipperiness to general-purpose films, inert inorganic compounds (e.g., silica, clay, titania, etc.) or organic compounds (e.g., calcium terephthalate, or high-melting polyester when polyester films are to be treated) have been conventionally added and/or there have been used methods in which a large number of fine protrusions have been formed on the surfaces of the films by utilizing the residues of catalysts. Alternatively, films containing inert inorganic compounds added thereto have been monoaxially or biaxially oriented, under heating, to increase the number of the protrusions formed on the surfaces thereof or to control the heights of the protrusions. In these methods, increased slipperiness can also be imparted by reducing the coefficient of friction of the surfaces of the films. Reduction in the coefficient of friction of the surfaces of the films can be effectively accomplished by forming many surface protrusions which are high and large. When these methods are applied to aromatic polyester films, unfortunately, the turbidity of the films is increased, transparency is lowered and optical characteristics deteriorate.
The present inventors have studied such problems and found that there can be obtained an aromatic polyester film having improved surface slipperiness property without detrimentally affecting the optical characteristics thereof. The present invention is based on this finding.